The "liquid biopsy" test identifies defective fragments of DNA shed into the bloodstream by cancer cells as they die.
Scientists have already used it to detect genetic faults involved in tumour growth in blood samples from 20 women with ovarian cancer.
They were also able to build a real-time picture of how one woman's breast cancer responded to treatment over more than a year.
Study leader Dr Nitzan Rosenfield, from Cancer Research UK's Cambridge Research Institute, said: "This type of blood test has the potential to revolutionise the way we diagnose and treat cancer. The great advantage is that it can be used to identify cancer mutations without surgery or a biopsy, making it safer and cheaper."
In future, patients could be given treatments based on the result of a quick blood test, rather than having a tissue sample surgically removed for analysis.